HgBr 2 : an easily growing wide-spectrum birefringent crystal.

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr 2 ) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34-22.9 μm). HgBr 2 has an exceptionally large birefringence (Δ n , 0.235 @ 546 nm), which is 19.6 times that of commercial MgF 2 . The ordered linear motif [Br-Hg-Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr 2 can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr 2 crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.